Axle and method of making the same



Feb. 28, 1956 R. K. POTTER AXLE AND METHOD OF MAKING THE SAME Filed Aug.25, 1952 United States Patent AXLE AND METHOD oF MAKING THE SAME RodneyK.' Potter, Hobart, Ind., assignor to United States Steel Corporation, acorporation of New Jersey Application August 25, 1952, Serial No.306,247

2 Claims. (Cl. 148-12) This invention relates to improvements of the artof quenching axles to impart residual compressive stresses therein.

Wide experience of railroads has amply demonstrated a superiorperformance of axles used in rolling stock after these axles have beengiven a treatment which induces compressive stresses in the layers ofmetal constituting the outside shell of the axles.

A conventional manner of imparting such stresses in thev outer layer ofaxles is set forth in tbe Making, Shaping and Treating of steel by C. B.Francis, 5th edition, pages 819-821. This practice consists in oil orwater quenching steel axles from above their upper critical temperaturefollowed, if desired, by a drawing treatment at a temperature below theupper critical point. Theoretical reasoning underlying the selection ofsuch treatments was based on the assumption that the transformation ofaustenite to martensite on quenching causes compression, sincemartensite has a larger space lattice parameter than austenite.

Quenching or quenching and drawing provided mproved results, but at thecost of certain negative features. Railroad axles and similar bodies areconventionally forged and roughly machined before heat treatment, andthen turned down on a lathe to the linal accurate dimensions. Areasonably high carbon content used for providing axles with thenecessary strength develops an excessive hardness when in quenchedcondition which renders machining operations somewhat dicult. When adrawing operation follows the quenching treatment in order to bring thehardness to the desired machnability range, the drop in compressionalstresses present in the outside layers of axles is frequently sucientlylarge to affect markedly the benelicial results produced by th originaltreatment.

I have discovered that the mechanism of stress formation previouslyrelied upon for treating axles, which depends on allotropictransformations of steel, is not necessary and that superiorfatigue-resistance properties and improved machinability can be impartedto axles by quenching them from below their lower critical temperature.The subcritical quench produces the proper stress pattern withoutmaterially altering the level of hardness in as forged and doublenormalized axles.

Accordingly, it is an object of the present invention to provide animproved method of heat treating axles.

It is a further object to provide a method of heat treating axles toproduce properly balanced stresses therein.

It is another object to provide an axle having enhanced resistance tofatigue.

The foregoing and further objects will be apparent from the followingspecification when read in conjunction with the single figure of thedrawings which is a graph of the stresses obtained by my improvedtreatment and by prior art methods.

I have found that the best resistance to fatigue failure can be providedby producing at the surface of the axles residual compressive stressessuch that the sum of residual ffice surface stresses and those developedon the compression side of the axle by loading in service do not exceedthe compressive yield point of the steel used for axles. Furthermore,the surface compressional stresses should be sutliciently large toprevent the algebraic sum of the surface compressional stresses and ofthe tensional stresses induced on the tension side of the axle byloading from exceeding 10,000 pounds per square inch in tension. Suchaxles may be produced from steel containing from .30 to 50% carbon, 50to 80% manganese, .02% maximum phosphorus, .04% maximum sulphur and .30%maximum silicon with the balance iron.

During a comprehensive search for means suitable for producing thedesired stress distribution in axles it was discovered that conventionalheat treatment is not satisfactory. Table I presents some of the resultsrecorded in applying conventional treatment, namely quenching from abovethe upper critical point either in water or in oil, and tempering tosections of seven-inch diameter cylinders machined to size from axlescontaining 0.46% carbon,v0.68 manganese, 0.016% phosphorus, 0.028%sulphur, 0.23% silicon. The Aci point of the steel was 1353 F. and theAri point was 1235 F. These cylinders were heated in a neutralatmosphere to 1600 F. until thoroughly soaked, quenched in oil or inwater, and drawn at different temperatures for varying times.

Heat treated cylinders were studied by appropriate methods for stressdistribution, only the calculated longitudinal stress being presentlyreported in detail.

Calculated Longitudinal Stress, lbs./sq. in.

Brlnell Time 0i Hardness Type of Cylinder g Draw` Water 011 Hollow, openend A corresponding set of figures obtained on specimens subjected -toquench alone without any subsequent drawing is given in Table Il, inwhich the treatments employed were strictly comparable to those used incollecting data given in Table I.

TABLE II Test cylinders quenched only Calculated Longi- Qunch- Hardnesstulcinl Stress,

g s. sq. in. Type of Cylinder TgmFp.,

- Water Oil Water Oil Hollow, open end 1,600 51,000 Hollow, plugged en1, 600 305 243 104, 000 80, 000 Do 1,300 193 158 60,000 60,000 Solidl,300 195 72,000 Do..- 1,000 180 47,000 35,000 Do 900 37, 000 D0 700 1762, 000

A comparison of values given in the above tables can be preferably madeinthe light of practical experience which indicates optimumfatigueproperties being asso ciated with a surface compression betweensubstantially 30,000 and 40,000 pounds, per square inch for a steelhaving a yield point of about 70,000 andthe requirement of a hardnessbelow about 200 Brinell for satisfactory machining. Viewed from thisstandpoint, conventional quenching followed by drawing does not producethe desired combination or" surface compression and hardness, while asingle subcritical quench can develop themV when applied within aspecific range of factors. A cylinder quenched in water from 1000 F.gives a surface compression of about 47,000, which is too high, andWater quenching from 700 F. causes a surface compression of about 22,000pounds per square inch which'is too low to be acceptable. Waterquench-ing from 900 F. produces a surface compression of about VIV37,000together with a satisfactory hardness, thus indicating a preferred waterquenching practice at temperature above 700 F. and below l000 F.

in case of oil quenching,it has been found that. quite satisfactorysurface compression of 32,000- pounds per square inch and good hardnesscan be effected by quenching from 1000" F. in oil, and accept-ableresults can be expected when the oil quenching range extendssubstantially from 800 to ll-F.

It has been found, furthermore, that a pronounced eduction in hardnessand improvement inrnachinability of axles treated by the method of thepresent invention does not affect adversely the stress distribution onthe surface of the axles. in the single figure of theV attached drawing,longitudinal compression and tension stresses in axles heat treated inthe conventional manner and according to the present invention areplotted as a function of distance from the center of shafts. It can beseen therefrom that quenching in oil from 1600 Efollowed by drawing at800 F. produces compression of 32,000 pounds per square .inch on theoutside surface. Axles quenched in oil from l000 F. show a stresspattern which compares very favorably with thev pattern produced by oilquenching from 1600" Fyand drawing at 800 F. developing a surfacecompression of 35,000 pounds lper square inch. Likewise, water quenchingfrom 900 F., produces stresses of approximately `the same level, theactual compression value in this case being 37,000 pounds per squareinch. A reference to Table I will `show that the Brinell hardnessofaxles-quenched at 1600D F. in oil and drawn at 800 F. is 250, which istoo high for good m-achinability, while Table II shows that the oilquenched and Water quenched cylinders have a Brinell hardness of 1530and 190, respectively, both remaining in good machinability range.

The present method offers therefore means for producing heat treatedaxles which have the fatigue resistance characteristics substantiallyequal to the best obtainable by conventional methods, but which, at thesame time, possess improved machinability unavailable by the prior artmethods.

While I have shown and described several specific embodiments of myinvention, it will be understood that these embodiments are merely Aforthe purpose 4of illustration and description and that various otherforms may be devised within the scope of my invention, as dened in theappended claims.

I claim:

1. The method of producing railway car axles having enhanced resistanceto fatigue, comprising forming a roughly machined forged blank of steelcontaining .30to .50% carbon, .50 to .80% manganese, .30% maximumsilicon with the balance iron and residual impurities, heating sai-dIrough machined blank to a subcritical temperature between 700 and l F.,then quenching said heated blank topr-ovi-de aBrinell hardness valueless than 200 and residual surface stresses which combined with thestresses developed on the compression side of the axle during loadingare less than the compression yield point of the steel, and thenmachining said quenched blank to its lfinished size.

2. A railway axle produced in accordance with the process described inclaim l.

References Citedin the tile of this patent UNITED STATES PATENTS `CoinMar. 12, 1889 Longford Aug. 13, 1929 OTHER REFERENCES

1. THE METHOD OF PRODUCING RAILWAY CAR AXLES HAVING ENHANCED RESISTANCETO FATIGUE, COMPRISING FORMING A ROUGHLY MACHINED FORGED BLANK OF STEELCONTAINING .30 TO .50% CARBON, .50 TO .80% MANGANESE, .30% MAXIMUMSILICON WITH THE BALANCE IRON AND RESIDUAL IMPURITIES, HEATING SAIDROUGH MACHINED BLANK TO A SUBCRITICAL TEMPERATURE BETWEEN 700 AND 1100*F., THEN QUENCHING SAID HEATED BLANK TO PROVIDE A BRINELL HARDNESS VALUELESS THAN 200 AND RESIDUAL SURFACE STRESSES WHICH CONBINED WITH THESTRESSES DEVELOPED ON THE COMPRESSION SIDE OF THE AXLE DURING LOADINGARE LESS THAN THE COMPRESSION YIELD POINT OF THE STEEL, AND THENMACHINING SAID QUENCHED BLANK TO ITS FINISHED SIZE.